In a typical internal combustion engine, the energy generated from combusting fuel is used to rotate a crankshaft. The crankshaft rotational force or torque is transmitted to one or more camshafts by a synchronous endless drive arrangement, typically referred to as a timing drive, which includes toothed rotors connected to the crankshaft and camshafts and an elongate drive structure such as a chain or toothed belt to interconnect the crankshaft rotor to the camshaft rotors and rotate them substantially synchronously with one another. The camshafts include cams that selectively open and close intake valves and exhaust valves. The intake valves allow air (and in some engines, fuel) to ingress and fill the combustion chambers of the engine. (In some engines the fuel is directly injected into the combustion chamber through a fuel injection valve rather than the air intake valve.) The air-fuel mixture is then combusted to generate power to rotate the crankshaft. After combustion, exhaust gas is discharged from the combustion chambers through the exhaust valves when they are opened by the corresponding cams.
The intake and exhaust valves and the camshafts that open and close these intake and exhaust valves generate a particularly intense source of mechanical vibrations. The torque of a camshaft fluctuates when it selectively opens and closes the corresponding intake valves or exhaust valves. The intake valves and the exhaust valves are constantly urged by valve springs in a closing direction. When the valves are opened against the force of the springs, torque opposite to the direction of camshaft rotation acts on the camshaft. On the other hand, when the valves are closed, torque in the rotating direction of the camshaft acts on the camshaft. This torque fluctuates the mean or nominal torque required to drive the camshaft and corresponding camshaft rotor and leads to a type of vibration known as torsional vibration or timing error—a variation in the angular position of the camshaft relative to the crankshaft. This occurs because the chain or belt is not entirely rigid but has sufficient elasticity to enable the camshaft rotors to overshoot or undershoot their ideal synchronized positions. When the frequency of the valve torque fluctuations is close to a natural frequency of the timing drive, system resonance occurs. In resonance the camshaft torsional vibrations and the span tension fluctuations of the chain or belt are at their maximum. These effects reduce the service life of the chain or belt and waste energy.
A number of prior art disclosures such as JP H01-95538, DE 19812939, JP 62-192077 attempted to reduce timing drive span tensions by introducing a non-circular, e.g., elliptical or oval, rotor into the timing drive. This prior art positioned the non-circular rotor so as to attempt to equalize tension, i.e., when the span tension was cyclically high (prior to installing the non-circular rotor), the eccentricity and angular position of the non-circular rotor was supposed to reduce span tension and when the span tension was cyclically high (prior to installing the non-circular rotor), the eccentricity and angular position of the non-circular rotor was supposed to increase span tension.
The prior art solutions which attempted to equalize tension do not work and actually make the problem worse.
This can be appreciated from first principles. Generally speaking, resonance occurs in a vibrating system when it is excited by an oscillatory force having a frequency close to a natural frequency of the system. For example, a mass at the end of spring will oscillate when the system is shaken by an oscillating external force. When the shaking occurs at a frequency equal to the natural frequency of the system, the response motion of the mass will be 90 degrees out of phase with the oscillating force and cause the amplitude of mass movement to increase to a maximum.
A similar situation occurs in automotive timing drives as described by the simple physical model of FIG. 1A. The vibrating component is a camshaft rotor of radius R, which has a particular moment of inertia J. The spans of the elongate drive structure provide the elasticity k and damping D in the system. The fluctuating valve torque T(t)=Tv*sin(ωt) is an externally applied oscillating force that acts on the camshaft rotor, causing cam torsional vibration. The system equation is T(t)=J{umlaut over (θ)}+D{dot over (θ)}+Kθ.
Such a resonant system behaves as shown in the frequency domain graphs of FIG. 1B.
The upper graph 4 of FIG. 1B shows the amplitude of the response or system output, such as span tension or cam torsional vibration. The response is highest for engine speeds close to the natural frequency.
The lower graph 6 of FIG. 1B shows the phase angle of the system response or output, such as span tension or cam torsional vibration, relative to the oscillating valve torque. As will be seen the phase angle varies considerably over the operating speed range and near resonance the response, i.e., cam torsional vibration (measureable in degrees of timing error) or span tension (measureable in Newtons), is 90 degrees out of phase with the fluctuating valve torque. FIG. 1C shows a corresponding relationship plot 8 in the time domain between fluctuating valve torque 8a and span tension 8b over time at an engine speed corresponding to the natural frequency of the timing drive.
The phenomenon of resonance causes the magnitude and phase of span tension and cam torsion vibration to fluctuate considerably, resulting in the significant variation of these parameters over engine speed as shown in a time domain simulation 9 of FIG. 1D, which plots tight side chain tension (in Newtons) for one rotation (in degrees) of the crankshaft rotor at different engine speeds for a particular 4-cylinder dual-overhead cam (DOHC) engine. As will be seen, at each engine speed the chain tension varies cyclically over one complete rotation. In this particular example, the maximum tension (about 1300 N)—and resonance—occurs at an engine speed of about 3000 rpm. Note also that angular position of peak tension varies over engine speed, i.e., the phase of the cyclical tension curve varies over engine speed.
The prior art which sought to equalize tension in the timing drive did so at an inopportune time and with an incorrect force. As seen in FIG. 1C, when the tension 8b is at a maximum, the fluctuating valve torque 8a is at zero. Conversely, when the fluctuating valve torque 8a is zero the tension 8b is at a maximum. Attempting to equalize tension, which is an output of the resonant system, does not address the fluctuating valve torque, which is an input of the resonant system. Indeed, the problem is exacerbated. As shown in FIG. 1E, when, for example, the fluctuating valve torque T(t) is at a maximum Tv in a direction (counter-clockwise) opposite to the direction of rotation (clockwise), the concept of tension equalization would orient the non-circular rotor at this instant as shown as this orientation would appear to alleviate the tension. However, tension and torque are not in phase so addressing the former does not address the latter. Instead, orientating the non-circular rotor in this manner introduces an additive fluctuating torque as schematically illustrated in FIG. 1E because the non-circular rotor pulls or stretches the elongate drive structure on one (left) side to generate marginal tension FL (in comparison to the situation when a circular rotor is installed) and relieves or relaxes the elongate drive structure on the other (right) side to generate marginal tension FR (in comparison to the situation when a circular rotor is installed). The marginal tensions generate an additive fluctuating torque TC that reinforces the fluctuating valve torque T(t), causing the system input excitation to increase. The end result is that the outputs of the resonant timing drive, such as span tension fluctuation and cam torsional vibration, also increase.
In U.S. Pat. No. 7,232,391, entitled “Synchronous Drive Apparatus with Non-Circular Drive Elements”, the contents of which are incorporated herein in their entirety, Gajewski of Litens Automotive realized that the “best cure” for cam torsional vibrations and span tension fluctuations caused by the opening and closing of the intake and exhaust valves, (which function as a source of torque fluctuations that cause the camshafts to be inflicted with speed fluctuations, which in turn, cause angular position fluctuations/torsional vibrations) is to attack the cause right at the source by introducing another torque acting on the camshaft through the use of a non-circular rotor which, while rotating, introduces fluctuations of span length by pulling and relieving the elongate drive structure n times per crankshaft revolution (n being related to the specific shape of the non-circular rotor and its position in the timing drive) such that when the tight side of the elongate drive structure is pulled the slack side thereof is relieved, and vice versa. The pulling and relieving of the elongate drive structure generates a new additional fluctuating torque at the camshaft to balance or counteract the fluctuating valve torque. Continuing with the previous example, as shown in FIG. 1F, when the fluctuating valve torque T(t) is at a maximum Tv in a direction (counter-clockwise) opposite to the direction of rotation (clockwise), Litens taught to orient the non-circular rotor at this instant as shown to pull or stretch the elongate drive structure on one (right) side to generate marginal tension FR and relieve or relax the elongate drive structure on the other (left) side to generate marginal tension FL. Although this is counter-intuitive because it appears to increase belt tension, the Litens solution is based on correct dynamic reasoning. The marginal tensions generate a corrective fluctuating torque TC that counteracts or works against the fluctuating valve torque T(t) to thereby reduce the system input excitation, with the end result being that the outputs of the resonant drive, such as span tensions and cam torsional vibration, are decreased. In short, the Litens solution works because it attacks the source of the disease.
Litens also contributed to the state of the art by recognizing that the dominant force generated by non-circular rotors in automotive timing drives arises from fluctuations of span length caused by pulling and relieving the elongate drive structure; forces governed by Hookes Law. Forces generated by other physical effects, such as changes in moment arm, are negligible and implementation based primarily on such effects leads to nonsensical eccentricities in automotive timing drives.
Litens recognized that the non-circular sprocket could be applied to counteract any source of torsional vibration in a synchronous system, as the solution could be applied to counteract any driven rotor coupled to a rotary load assembly which presents a periodic fluctuating load torque when driven in rotation. For example, the timing drive can also drive other components which cause mechanical vibrations, such as a fuel injection pump. A fuel pump can convert rotation of a pump rotor into reciprocal motion to reciprocate a piston. The reciprocation of the piston introduces fuel from a fuel tank into a pressurizing chamber of the pump. The piston then pressurizes the fuel and supplies the fuel to the fuel injection valves. The fuel injection pump applies a reactive force on the pump rotor. The magnitude of the reactive force during its suction stroke is different from the magnitude during its compression stroke. In other words, the magnitude of the reactive force fluctuates. Therefore, the actuation of the fuel injection pump can also cause a fluctuating reactive torque at the pump rotor, leading to another source of torsional vibration in the timing drive system.
Furthermore, Litens disclosed that the resonant system input excitation could derive from multiple sources. For example, in a DOHC engine where the timing drive drives each of the intake and exhaust camshafts, the concatenation of the fluctuating valve torques generated from both camshafts presents a fluctuating load torque that can be counteracted by a non-circular rotor. Likewise, in an engine where a fuel pump is present, a combination of the fluctuating fuel pump torque and fluctuating valve torques presents a fluctuating load torque that can be counteracted by a non-circular rotor.
Litens thus presented an apparatus and method of reducing torsional vibrations in a synchronous drive apparatus having a continuous-loop elongate drive structure with a plurality of engaging sections (such as belt teeth or chain links), a plurality of rotors including at least a first and a second rotor, wherein the first rotor has a plurality of teeth for engaging the engaging sections of the elongate drive structure, and the second rotor has a plurality of teeth for engaging the engaging sections of the elongate drive structure; wherein the elongate drive structure is engaged about the first and second rotors, the first rotor being arranged to drive the elongate drive structure and the second rotor being arranged to be driven by the elongate drive structure; and wherein a rotary load assembly is coupled to the second rotor such as to present a periodic fluctuating load torque when driven in rotation. One of the rotors has a non-circular profile having at least two protruding portions alternating with receding portions, wherein the angular positions of the protruding and receding portions of the non-circular profile relative to the angular position of the second rotor, and the magnitude of the eccentricity of the non-circular profile, are such that the non-circular profile applies to the second rotor an opposing fluctuating corrective torque which reduces or substantially cancels the fluctuating load torque of the rotary load assembly.